1. Field of the Invention
The present invention relates to methods for manufacturing a display device, and laminated structures.
2. Description of the Related Art
In addition to having the display quality at least equivalent to conventional cathode ray tubes (CRTs), active matrix liquid crystal displays (TFT-LCDs), which use semiconductor thin film transistors (TFTs) as driving devices, have characteristics such as a thin thickness, light weight, high definition, low power consumption, and an ability to increase the display size, as compared to the CRTs. Due to these characteristics, the TFT-LCDs are currently used in various fields as next generation displays that replace the CRTs. Organic electroluminescence (EL) displays and the like have also attracted attention as next generation display devices in recent years. Although many of such image display devices are currently formed on glass substrates, it is expected that, in the future, the image display devices will be used as flexible displays by using plastic substrates, stainless steel substrates, and the like. Such flexible displays are advantageous since they are not broken even when folded or rolled, and can be carried in a compact form.
In order to form the flexible displays by the technologies of LCDs, organic EL, and the like, plastic, stainless steel, and the like need to be used as substrate materials. Due to their flexibility, it is generally very difficult to carry and process such substrates by conventional process apparatuses for processing glass substrates. Thus, a major remodeling is required to enable the conventional process apparatuses for glass substrates to process plastic substrates and the like. However, this requires enormous investments, which result in a price increase of final products. In a conventional method developed as a solution to these problems, a plastic substrate is carried and processed after being bonded to a glass substrate, and is removed from the glass substrate after fabrication of devices.
A delamination transfer method is also proposed in Japanese Published Patent Application No. H10-206896 and the like, as a method of fabricating thin film devices on a glass substrate, transferring only the device layer onto a temporary substrate, and then, transferring the device layer again onto a plastic substrate. In this method, a delamination layer of hydrogenated amorphous silicon, and a protective layer are first formed on a glass substrate, and then, devices are fabricated by a commonly used process. Then, a weak adhesive layer is formed on the device surface to bond a temporary transfer substrate to the glass substrate. Then, the back surface of the glass substrate is irradiated with laser light, whereby hydrogen is generated from the delamination layer. Thus, the device layer delaminates from the interface between the delamination layer and the protective layer, and the device layer is transferred in an inverted fashion onto the temporary transfer substrate. Then, a permanent adhesive is formed on a flexible substrate, such as a plastic substrate, and the device layer of the temporary transfer substrate is bonded to the flexible substrate. Thereafter, the temporary transfer substrate is delaminated from the weak adhesive layer, whereby delamination and transfer of the device layer onto the plastic substrate is completed. In this laser delamination transfer method, devices are fabricated on conventional glass substrates. Thus, a major advantage of the laser delamination transfer method is that this method not only enables the apparatuses for glass substrates to be used, but also enables exactly the same high temperature process as in conventional examples to be used without the need to consider the problems that occur when using the plastic substrates, such as the heat resistance and the water absorbing property.
In order to fabricate devices such as TFTs on a flexible substrate, such as a plastic substrate, bonded to a glass substrate, strong adhesion is required to prevent delamination of the flexible substrate during a chemical solution process or a high temperature process. Actually, conventional adhesives are progressively cured by the heat history when subjected to a high temperature process several times, whereby the adhesion strength increases more than intended. Thus, the adhesion strength often becomes higher than in the initial state by the time fabrication of the TFT devices is completed. However, it is very difficult to delaminate the strongly bonded plastic substrate after fabrication of the devices. Forcibly delaminating the plastic substrate can partially break the substrate, or can damage the devices formed thereon.
The use of adhesives that are not resistant to heat and chemical solutions also has a problem. Adhesion strength of the adhesives is reduced by a heat treatment and a chemical treatment, and the plastic substrate delaminates from the glass substrate during the process.
In the laser delamination transfer method proposed as one of the methods for delaminating and transferring a device layer onto a plastic substrate, a hydrogenated amorphous silicon layer, which is formed as a delamination layer directly under the device layer, is irradiated with laser light via the back surface of a glass substrate. The device layer is delaminated from the glass substrate, and at the same time transferred onto a temporary transfer substrate by using the desorption force of hydrogen. However, a thin film having an overall thickness of about several micrometers is actually very fragile, and it is very difficult to stably delaminate the thin film from the glass substrate. For example, if the delamination layer is not sufficiently irradiated with laser light due to foreign matter, small scratches on the back surface of the glass substrate, or the like, the adhesion strength of that part is not reduced, and the device layer can be easily chipped when the temporary transfer substrate is delaminated from the glass substrate. A similar problem can occur when the hydrogenated amorphous silicon as the delamination layer varies in quality and thickness. Energy stability of the laser itself is also a problem. The laser delamination transfer method usually uses excimer laser that is easily absorbed by hydrogenated amorphous silicon. However, the radiation energy of the excimer laser itself is unstable, and the energy radiation significantly varies every time the excimer laser is used. Moreover, it is difficult to control the desorption force of hydrogen from hydrogenated amorphous silicon. Thus, as described above, insufficient irradiation with the laser light does not sufficiently reduce the adhesion strength, causing chippings and defects of the device layer. Excessive irradiation with the laser light can break the device layer due to a too strong desorption force of hydrogen, and can cause the device layer to be directly damaged by the laser light that has passed through the delamination layer.
Moreover, even after the delamination layer is transferred onto the temporary transfer substrate, any residue of the delamination layer adhering to the device transfer layer needs to be removed. At this time, the device layer, bonded to the temporary bonding substrate with weak bonding strength, is easily broken as it is repeatedly subjected to a dry etching process and a wet cleaning process. Moreover, one of the major problems of the delamination transfer method is that the delamination layer needs to be delaminated and transferred to the plastic substrate when the TFT process is completed. LCD manufacturing processes include a liquid crystal process in which a liquid crystal material is enclosed in a liquid crystal cell after TFT devices are fabricated. However, in the delamination transfer method, the device layer needs to be held on the temporary transfer substrate when the device layer is transferred. Thus, the delamination transfer process cannot be used for the liquid crystal process in which the substrate and a counter substrate are bonded together. Thus, a separate technique of stably carrying a soft plastic substrate is therefore essential.
A method of etching the glass substrate itself to remove the device layer has also been proposed in the delamination transfer method. However, etching of the glass substrate requires a very strong hydrofluoric acid solution. Since such a hydrofluoric acid solution easily damages the device layer, a strong barrier thin film is required. However, if the barrier film has defects, the device layer is extensively damaged by an etchant that has passed through the barrier film.